1. Field of the Invention
The present invention relates to improvements in wide band gap power modules. More particularly, the invention relates to improvements particularly suited for providing a configurable consistent power module design for multiple applications. In particular, the present invention relates specifically to a parallel path power module allowing for current sharing at high switching frequencies.
2. Description of the Known Art
As will be appreciated by those skilled in the art, power modules are known in various forms. Patents with information of interest to power modules include: U.S. Pat. No. 7,687,903, issued to Son, et al. on Mar. 30, 2010 entitled Power module and method of fabricating the same; U.S. Pat. No. 7,786,486 issued to Casey, et al. on Aug. 31, 2010 entitled Double-sided package for power module; U.S. Pat. No. 8,018,056 issued to Hauenstein on Sep. 13, 2011 entitled Package for high power density devices; U.S. Pat. No. 8,368,210 issued to Hauenstein on Feb. 5, 2013 entitled Wafer scale package for high power devices; U.S. Pat. No. 6,307,755 issued to Williams, et al. on Oct. 23, 2001 entitled Surface mount semiconductor package, die-leadframe combination and leadframe therefore and method of mounting leadframes to surfaces of semiconductor die. Additional articles include: R. K. Ulrich and W. D. Brown, “Advanced Electronic Packaging,” New Jersey: John Wiley & Sons, Inc., 2006, p. 203; and Shengnan Li, “Packaging Design of IGBT Power Module Using Novel Switching Cells,” Ph.D. dissertation, University of Tennessee, 2011, http://trace.tennessee.edu/utk_graddiss/1205. Each of these patents and publications are hereby expressly incorporated by reference in their entirety.
Wide band gap power semiconductors, including Silicon Carbide, SiC, and Gallium Nitride, GaN, offer numerous advantages over conventional Silicon, Si, based power electronic devices, including:
1. Reduced intrinsic carriers allowing for higher temperature operation
2. Increased carrier mobility
3. Higher electrical breakdown strength
4. Reduced on-resistance
5. Faster switching speeds
6. Increased thermal conductivity
These benefits allow for designers to implement systems which are substantially smaller, more efficient, and more reliable that the current state-of-the-art systems. Higher temperature operation allows for the reduction of the cooling system required to remove waste heat. The potential also exists to switch from an active, i.e. forced air or liquid, cooling scheme to passive, natural convection, cooling, elimination of thermal shielding materials, and operation in extreme environments where traditional technology will fail. High frequency switching reduces switching losses and allows for a major reduction in the size of filtering elements in a power converter.
The promises of wide band gap power technology, however, are hindered by the power packaging necessarily to interconnect, protect, and integrate the devices into a power conversion system. Power packages for Si devices are generally designed to house one large device per switch position, often with a single antiparallel diode. Commercially available wide band gap devices, however, are not available as large, monolithic elements due to issues with wafer quality and yield. Accordingly, while the relative power density, per die area, for SiC is substantially higher than Si, in order to reach high currents, in the hundreds of amps, many SiC devices must be placed in parallel.
There is a fundamental issue with paralleling many devices in conventional packages which were not designed to effectively account for issues such as current sharing. This is particularly important due to the extremely high switching speeds of wide band gap devices, often hundreds of times faster than Si equivalents. Mismatches in inductances between the devices may cause uneven stresses and current overshoot during switching events. Additionally, the materials, attaches, and interfaces of established power module technology are not capable or reliable at the temperatures which wide band gap devices are operable.
From this, it may be seen that the prior art is very limited in its teaching and utilization, and an improved power module is needed to overcome these limitations.